It's often an upstream issue, and I'm not keen on people who suggest men in middle age (40s, 50s) should automatically jump to supplementing testosterone. I had low T for a while and it turned out it was due to a vitamin B deficiency (I'm a T2 Diabetic and my medication basically leeches vitamin B from my body). Started supplementing and the next two tests over several months went from low, to the lower end of "normal" range, and seemed to be steadily increasing back to normal.